haskell - Pattern matching identical values -

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I was wondering if functional programming languages ​​(Hassel / F # / lotus).

Just think about the following example: 

  plus aa = 2 * a plus ab = a + b

the first version Will be called when the function is applied with two equal values ​​(which are stored in a ).

A more useful application would be (Simplification) an AST). 

  simple (add AA) = multi 2 one

but Haskell rejects these codes and gives me the for conflicting definitions Warning A - To know whether the function gets the same value or not, I have a clear case / if-check to indicate whether there is a move to indicate that one variable Against that I want to match which will happen many times?

This is called an nonlinear pattern about it's Haskel cafe mailing list But there are many threads, not long ago. Two below:

Bottom row: Applying is not impossible, but for simplicity a decision has been taken against it.

By the way, you do not need to work around if or case ; To use a guard (a little bit) cleaner is the way: 

  one plus' b. A == B = 2 * A. Otherwise = A + b

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